270 REPRODUCTION, EVOLUTION, ETC. 



studded with pores through which the threads passed. It is known 

 from the Ordovician and Silurian. 



Cyclocrinus (fig. 172) is a genus which grew to about seven 

 centimetres and looked like a miniature golf ball borne on the end 

 of a stalk. Narrow branches arose at the apex of the stalk and each 

 terminated in a flattened hexagonal head, but as the edges of 

 adjoining heads were fused together to form the outer membrane, 

 which was only weakly calcified, the cell outlines were clearly 



Fig. 172. Dasycladaceae. Cyclocrinus porosus (x 8). (After Hirmer.) 



visible. Many species are known from the Ordovician and Silurian, 

 all somewhat resembling the living genus Bornetella. 



Primicorallina (fig. 173), from the Ordovician, had a segmented 

 stem beset with radially arranged branches, each of which branched 

 twice into four branchlets. 



The type of structure found in Diplopora (fig. 174) was also 

 shown by many other forms from the Middle Triassic. It was a 

 few centimetres long and bush-like in appearance, the main stem, 

 which sometimes had a club-shaped apex, being covered with 



